THE ROYAL ARTILLERY INSTITUTION. 
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This principle is partially carried out:— 
In Armstrong's B.L. guns by giving, through means of shrinking, 
greater tension to the outer coils than to the inner ones, so that the former 
do a certain amount of work in compressing and energetically supporting 
the latter, and still more in his M.L. guns by the employment in addition 
of a stronger material (steel) for the inner barrel. 
In Whitworth's, where a similar effect is produced by forcing on the 
outer tubes by hydraulic pressure. 
In Palliser's, by making the barrel of a stronger material than the 
outside. 
In the Eodman plan of casting adopted by the United States in which 
the hollow casting is cooled from the interior, so that the inner portion is 
compressed and supported by the contraction of the outer portion around 
it. 
The last method approaches nearest of all perhaps to the fulfilment of the 
principle as the metal becomes gradually stronger and stronger from the 
outside to the bore, whereas in all the other systems the strength of the 
metal does not progress gradually by regular increments but per saltum 
according to the number of parts superimposed. Unfortunately, however, 
cast-iron even though thus scientifically disposed is naturally too weak and 
brittle as a material for rifled ordnance; but it is not impossible that safe, 
powerful and cheap guns may yet be made in this manner of an improved 
description of steel. 
To Sir William Armstrong is due the merit of employing wrought-iron 
coils shrunk together. His main principles of gun architecture consist 
essentially:— 
First, In arranging the fibre of the iron in the several parts so as best 
to resist the strain to which they are respectively exposed; thus, the walls 
or sides of the gun are composed of coils with the fibre running round the 
gun so as to enable the gun to bear the transverse strain of the discharge 
without bursting, whilst the breech end is fortified against the longitudinal 
strain, or tendency to blow the breech out, by a solid forged breech-piece 
with the fibre running along the gun. 
Secondly, In shrinking the successive parts together so that not only is 
cohesion throughout the mass ensured but the tension may be so regulated 
that the outer coils shall contribute their fair share to the strength of the 
gun in accordance with the theory already explained. 
With regard to the first principle a gun may be destroyed either by the 
bursting of the barrel or by the breech being blown off. Now, wrought-iron 
in the direction of its fibre is about twice as strong as it is in the cross 
direction, hence the best way to employ it to resist the transverse strain is 
to wrap it round and round the piece like a rope. This is the foundation 
of the Armstrong coil system. For the same reason the best way to resist 
the longitudinal strain is to place the fibre lengthways or end on; so a 
